1. Field of the Invention
The present invention relates to a stationary position detection circuit and a Hall sensorless motor drive circuit capable of detecting the position of a motor rotor.
2. Description of the Background Art
In driving a motor having a rotor such as a small three-phase DC brushless motor, the motor drive circuit is required to be kicked at the time of starting. In the process, unless the rotor position can be detected properly, the proper starting is impossible.
The rotor position can be detected by arranging a Hall sensor configured of a Hall element in the neighborhood of the motor rotor. The use of the Hall sensor, however, leads to an increased cost and a bulkiness. Currently, therefore, vigorous efforts are made to develop what is called a Hall sensorless motor using no Hall sensor.
In the Hall sensorless motor, no induction voltage (counter electromotive voltage) is generated as long as the motor is stationary, so that the position of the rotor cannot be detected. As described in Japanese Patent Application Laid-Open Nos. 2002-345286, 2002-335691 and 2002-315385, therefore, a method has been developed in which the length of the kickback time for turn-off operation is detected by a stationary position detection circuit thereby to detect the position of the rotor of a motor in stationary mode.
Japanese Patent Application Laid-Open No. 2003-47280 is also available as another patent document related to the present patent application.
In the case where the kickback voltage at the time of turning off is measured as in the stationary position detection circuit described in Japanese Patent Application Laid-Open Nos. 2002-345286, 2002-335691 and 2002-315385, the difference in the kickback voltage value with a minuscule inductance difference depending on the rotor position while the motor is stationary makes it possible to detect the rotor position of a stationary motor by detecting the length of the kickback time.
In the detection method using the kickback voltage, however, a large kickback voltage is required to be generated by supplying a large kickback current (for example, about 1 A) to detect the minuscule inductance difference due to the difference of rotor position. This is by reason of the fact that a large kickback voltage is required to sufficiently recognize the difference in the length of the kickback time. The large kickback current is a cause of vibrations.
Also, in the detection method using the kickback voltage, the information indicating the inductance difference can be obtained only for the very short period during which the kickback occurs, and therefore the information may not be sufficiently detected.